Electronic switching telephone system



April 6, 1965 v. E. PORTER ELECTRONIC SWITCHING TELEPHONE SYSTEM 5 Sheets-Sheetl l Filed Jan. 24. 1961 April 6, 1965 v. E. PORTER f"3-177,291

ELECTRONIC SWITCHING TELEPHONE SYSTEM Filed Jan. 24. 1961 5 Sheets-Sheet 2 April 6, 1965 v. E. PORTER 3,177,291

ELECTRONIC SWITCHING TELEPHONE SYSTEM Filed Jan. 24. 1961 3 Sheets-Sheet .'5

United States Patent 3,177,291 f v ELECTRONIC SWITCHING TELEPHNE SYSTEM Virgie E. Porter, Country Club Hills, Ill., assigner to International Telephone and Telegraph Corporation Filed Jan. 24, 1961, Ser. No. 84,557 14 Claims. (Cl. 179-48) This invention relates to telephone `systems and more particularly to electronicswitching telephone systems.

In known telephone systems, a plurality of vertical and horizontal multiples are arranged to provide intersecting crosspoints, each multiple including message carrying and control conductors. Each crosspoint includes switching means for effectively coupling the message carrying conductors that intersect at the crosspoints. It has lsometimes been the practice also to include a memory or `control device at each crosspoint, which device is operated or released responsive to -a coincidence of signals on the control conductors that intersect at such crosspoint. These memory devices turn on or turn oil the switching means at the crosspoint, remember whether it is on7 or oi and control it accordingly.

The systems including' memory devices at each crosspoint have sometimes required expensive, unreliable components and complicated control circuits. r`For example, when electro-mechanical crosspoints are used, there is an extremely high level of electrical reliability -because open contacts provide an infinitely large resistance and closed contacts provide an inlinite'simally small resistance between the talking conductors.

However, 1

these same contacts are'not mechanically reliable and v `may openV or close if jarred or improperly mounted. Moreover, these contacts are sometimes subject to con lfl electronic crosspoints are p tarnination unless seale'din glass, and the glass may break under loer-tain conditions. used, thev resulting system may have electrical characteristicsUwhichm-'ake it dilicult to interconnect lelectronic switching systems with existing telephone equipment. To

overcome these and other problems, i-t has been necessary to provide extremely expensive installations with high maintenance costs, large power supplies, and coniplex maintenance problems, Iall ofwhich make it dit-ii-y cult to provide small, compact, portable telephone exchanges.

Accordingly, it is an object of this` invention' to pirovide new and improved electronic switching telephone systems andrmore particularly to provide improved crosspoints for use in such systems. Another object of this invention is to provide an integrated telephone system featuring line circuits and crosspoint matricesV which cooperate to provide telephone,

switching functions with a minimum number of components.

, Yet another object of this invention is to provide crosspoints having improved memory` deviceswhich control electronic switches and transmit busy indicating sig" nais when the crosspoint is closed. VIn this connection, n it is an object of this invention to provide improv-edv switching techniques utilizing the best featuresof magnetic and semi-conductor logic elements.

In accordance with one aspect of this invention, an

electronic switching telephone system is provided with a crosspoint matrix including horizontal and vertical multiples, cach multiple comprising message carrying and conrol conductors. rThe message carrying conductors which intersect at a crosspoint are eliectively coupled together by an electronic switching means. These electronic switching means operate during discrete time trames in conjunction witha magnetic device, here shown las a saturable reactor having a control winding connected be- 3,177,291 Patented Apr. 6, 1965 "ice tween the control conductors that intersect at a crosspoint.

In carrying out this invention, the saturable reactor is used to control the transmission ofy busy and idle line mar-kings.v More specifically, before acrosspoint tires the core flux of the saturable reactor is resting at Saturation in a first magnetic direction. During each time lframe which identities an associated station, the core flux. isy driven toward, but not to, saturation in the opposite direction. Substantially all energy applied during each time frame goes to change core flux and very little current flows through the Winding. When that time frame terminates, core linx is driven back to saturation in the [first direction. When a crosspoint ires,the corel lux is driven all the way to saturation in the opposite direction.

energy is dissipated by changing core ux and, avery large current flows as a busy marking.

prevents a double connection in the matrix.

The Aabove mentioned and other features and objects of this invention and the manner of obtaining them will becomemore apparent and the invention itself will be best understood .by'making reference to the following description of an embodiment of the invention ltaken in conjunction with the accompanying .drawings wherein:

FIG. y1 is a block diagram showing a telephone Sys"-- tem incorporating this invention;

FIG. 2 shows the logic symbols used elsewhere in drawings;

FIGS. 3-5 ar-e a schematic circuit diagram'showin'g;

details .of the circuit used to complete the hollow boxes of FIG. l;

FlGQSa is an idealized hysteresis loop trace which'is" helpful toward 'an understanding of how the. saturable"` reactor operatesgand Pro. 6 Shows how rios. 3-'5 should .be joined o1 provide a complete and understandable circuit.

Brief description' "As shown in FIG'. 1, an exemplary telephone systemy 1ncorporating this invention includesfainumber of` subscriber stations marked A-N, a pair of electronic crosspoint matricesA marked FINDER and CONNECT OR, and control equipment includinglinks, allotter, and cornmon highways. The allotter9' preassignseach idle link in turn toserve the next call extended through the exchange and Athen steps von to assign the next idle linkI after such call is'initiated. The common highways include an INPUT BUSY HIGHWAY, a LINK SEIZB HGHWAY, an OFF-HOOK HIGHWAY, and a.

cludes the control equipment necessary to complete an individual call. The horizontal andvertical'multiples are arranged to provide a number of intersecting crosspoints,

as shown at 14, 21, for exa'r'nple.vr

The time frames used to`l control the crosspoint switches emanate from a scanner l'which is a clock-like device that provides cyclically recurring pulses, each of which individuallyidentilies a subscriber station. scriber station A is identified by a rst pulse applied over Thereafter, during each time frame ,whichv identies an associated station substantially no applied' y The changein current flow' brought about bysaturation ofv the Core.

Thus, subidentified by a second and later pulse applied over conductor 16a, etc. TheV scanner also transmits al1 of its pulses over a cable 17 to each of the link circuits. Each linkr circuit is adapted to select a de siredone of the time frame pulses appearing on the cable 17 and, during that timeframe, to apply. a markingto the individually associated vertical multiples (such as multiples 12,18 in thel'- case of link circuit 13)'. If the -line circuit 11 is then marking the horizontal multiple 10, the crosspoint 14res'- and if the line circuit 19 is then marking the horizontal multiple 2.0, the crosspoint 21 tires. Other .crosspoints are red-in a similar manner. After crosspoints 14 and 21 tire, subscriber station A is connected to subscriber station B on a space switching basis via these tired crosspoints 14, 21 and the link circuit 13. Thereafter, and for the duration. of the call, the crosspoints 14, 21 transmit a busy indicating pulse to the link circuit 13 during each time frame that identifies subscriber stations A, B. Link circuit 13 applies a busy pulse to the INPUT BUSY HIGHWAY during these same time frames to prevent any other link from connecting with stations A, B.

YMeansare provided for preventing simultaneous connections to a single station via a plurality of links. More specifically, a seizeinhibit circuit 2.5 is interposed between the OFF- HOOK HIGHWAY and the link circuitsv to prevent simultaneous operation of more than oneV link circuit Vatany given time and, further, to prevent a link fromoperating during a time-frame which identities a y busy station. The circuit 25 operates in the following manner.

Before a call is placed by or extended to a specific station, no busy marking pulses are applied to the INPUT BUSY HIGHWAY during -the time frames` which identify that specific station. Therefore, the seizev inhibit circuit 25 pulses the LINKSEIZE HIGHWAY during such time frames if a call is extended from a callf igstation, as indicated by a pulseon the OFF-HOOK and vdescribeclby. a number ofl logic symbols which are individually shown in FIG; 2. More specically, an linhibit gateis shown by la semi-circle having. input, inhibit,

If: the inhibit terminal is enerf gized,ino current can vilowthrough the inhibit gate. If

and output terminals.

the inhibit terminal is not energized, any signals appearing on the input terminal are conducted through the gatev .to theoutput terminal are conducted through the gate to the.

' output terminal.

' An OR 'gateis shown by a semi-circle having a number of inputterminals which intersectrthe chord line.

E'nergizationof any input terminal of an ;OR gate;

causes a signal to appear on theoutput terminal.

HIGHWAY, thus preventing any other link from comtaneously, the registerV stage turns output conductor;l When the resetfterminal (marked by' For example, if a register stage A. is switched om it identities a subscriber station A. Likewise, if a register lstage N is turned on, it identifies a subscriber station N.. When the input and shift terminals are pulsed simul- 0n to Venergize the the heavily .inked dot) is -pulsed,'the stage turns off to deenergize .the output termina.; The circuit'details of this register are also shown in FIG. 4.

' A holdover amplifier is indicated by a rectangle having the letters HOA therein. This amplifier is de- Signedto provide a steady output signal if pulses recur at the input withinapredetermined time limit.. For one example of a devicesuch as this, referenceis made to a U.S. patent application VS.N.4 841,190, Ytiled by Ligotky on September 21, 1959, lnow U.S. PatentNo. 3,093,753, and assigned to the assignee ofthis invention.

Finally, a circuit sometimes called a Bip-flip or a flipflopis shown by a rectanglel bisected by aline. If the input terminal is pulsed, the flip-'filip turns on and energizes the output terminal -thereby .remembering the occurence of the input pulse.. This output terminal continues to be energized until the reset terminal (marked by follows, the y circuitry used to complete the system of FIG. l is shown `An,AND gate is showny by a` semi-circle including an ampersarid. When all input terminals rof an AND gteare energized simultaneously, a signal appearson an output terminal.

'Aftone gate is shownbya triangle enclosed within a circle. Any signal appearing on its input terminal is ap pliedto its output terminal if the switch conductor a heavily inked dot) is energized to turn olf theip-ilip and terminatethe output signal, thus, forgetting the oc-A currence of the input signal andrermernbering the occurence of the reset signal.

Detailed description into `the telephone switching system of FIG. -l.

The subscriber stations A, B yare shown; as the fourfwire variety which connectto switching equipment via va transmit pair of conductors TRNS and av receive pair L of conductors REQ y Each subscriber line is terminated by -an individually associated VlineV .circuit 11, 19- iricludingaa'pair of transformers TR1, TR2. The pri- .maries of these transformers are connected in a balanced circuit -to the Vsubscriber lines and the secondaries of 1these transformers are connected inan unbalanced circuit to electronic crosspointsy via the horizontal multiple,

10,1in the case of station;A'. An advantage-ofthe balanced external connection to the linev is. thatthe circuits tend to; maintain fidelity despite variation of line. impedance as Well as the 'avoidance vof ground loops. An advantage of the unbalanced internal connection is that the number of crosspoint switches required fis reduced lby one-half. y v u Lineciicin't. -The Vbalanced circuit of the transmit pair TRNS A diierence betweenthe transmit andrethrough the upper primary winding 500, and alainp 501 to a negative 4source ofpotentialy and through the'lower winding `502 to ground. Thus, the lamp 5015lights as an aid for maintenancepersonnel, if a short circuit occurs onV the line. ,These ground and-negative potentialsources provide the common talking .battery .(telephone'trans.-

mitter power').. Connected between `the `upper and lower primary .windings 500,. 502. :is a :capacitor 503 which isan A.C. shortcircuit at voicefrequencies so that the speech voltageappears almost entirely across thej transformerprimary winding..

The'unbalanced circuit mayv be traced from a negative battery connected to the lowerterminal of, the secondary winding 50.4 over a message Ycarryingconductor 10a to an emitter-collector` (i) of all electronic switches incorporated in theyassociated. horizontal 'multiplefof the switching matrices.

A matrix viajthe transformerY TR2 .1n a' mannerV similar vThe receive pairA REC is connected `to the crosspoint to that described .aboveinconnection with the transmit A difference between the. transmitand receiveQconr'iections is that a resistor 505 is connectedin series between the. lower primary lwinding 5061 and ground to providea current detector. Thatis, an oft-hook telephone at station A puts a D.C. short circuit across, the talking conductors of the telephone receive pair REC Thus, after subscriber station A goes olf-hook, the cury hook and ofi-hook supervisory signals. More specifically,

conventional open loop pulses are generated by a telephone dial connected across the receive pair REC at the subscribers station. When the digit one is dialed the D.C. loop across the receive pair REC is broken and reestablished once. When the digit two is dialed, the loop is broken'twice, etc. The time constant of the capacitor 516 .is such that it supplies a voltage to a switch amplifier 511 normally supplied over the completed talking conductors if the open loop condition does not persist for a time period which is longer than the normal dial pulse.

To provide for the detection of ott-hook conditions, the electronic switch amplifier 511 energizes an upper input of anr AND gate 512 and the lower input of an OR gate 515 when the D.C. loop is closed -across the receive pair REO The 'lower input of the AND gate 512 is energized over the conductor 16 which individually couples'the line circuit 11, to the scanner 470 and'which is energized duringa particular time frame that identities the subscriber station A. In thismanner, it'is apparentthat acoincidence between an ott-hook condition on the receive pair REC and of a pulse from 4the scanner during the'tiine .frame that identifies; the Vsubscriber station A results in 'an energization ofthe AND gate 512. The output' of AND gate 512 is applied through an -isolating diode 516 to the' OFF- HOOK highway to inform an 'assigned link of the occurrence of a call.

To extend a connection through the CONNECTORv matrix, another AND gate 517 has its lower input energized from the scanner via conductor'16 during the ytime frame which identities the associated subscriber The station A and its upper input energized by a pulse applied to the LINE SEIZE highway from a link circuit during the same timeframe. Thus, the coincidence of a seize pulse and a time frame pulse which identities the associated station A causesthe'AND gate 517 to energize a holdover ampliier 518. The holdover amplifier integrates the scan pulses and 'maintains a continuous output voltage on the control conductor 10c to'sustain the electronic crosspoint switches. Thus, the crosspoint vmatrix is supplied with sustaining voltage either when the switch amplifier 511 energizes the lower input of theY OR gate 515 responsive to an ofi-hook signal or when the holdover amplifier energizes the upper input of the OR gate 515 responsive to control signals from the link circuit.

Crosspoz'nt.-Next, reference is made to the details of the electronic crosspoint 14,.by way of example. The other crosspoints are the same. Uponinspection of the drawings,'it will beseen that the crosspoint 14 is located at the intersection of a iirst group 'of conductors lwhich form the horizontal multiple 1t) and a second group of f conductors which form the vertical multiple 12. The first group includes, as a speech path, a pair of message carrying conductors 10a, 10b, and a pair of control conductors 10c, 10d. The second group also includes, as a speech path, a pair of message carrying conductors 12b, 12C and a control conductor 12a.

As here shown, the horizontal multiple message carrying conductors 10a, ltib are connected frornthe secondary windings of the transformers TR1, TR2 in the line cirl6 Y cuit 11 to the emitter-collectors (i) of a pair of bilateral transistors 527, 52.3. One of the control conductors of the horizontal multiples is the HOLD conductor ltc which is energized either by the holdover amplifier 518 or by the switch amplifier 511 depending upon' whether the line circuit. serves a calling or a called line. Thus, as long as the'associated telephone station A is in an oli-hook condition or the line is in a called condition, an energizing potential is applied to the HOLD conductor by the line circuit. Also, the horizontal multiple includes another control conductor 10d marked SCA-N. This scan conductor is the one which extends from the scanner 470 through the associated line circuit 1l to provide a station identifying time frame. Thus, the SCAN conductor 10d is pulsed during each timeframe which identities station A. Both the HOLD and the SCAN lines extend across an entire horizontal multiple and lare eiiective at every cross-point of that multiple.

The vertical multiples include a pair of speech path conductors 12b, 12e, which connect directly to a corresponding pair 18h, tc in theconncctor matrix. Each vertical includes a control highway 12a, 18a which eX- tends to the individually associated link circuit 13. When an allotter preassigns allink to serve the next call, these v vertical control highways 12a, 18ct are energized from the link. When-a call is in progress these vertical control highways are pulsed through a iired'crosspoint during `each station identifying time frame.. The speech paths or message carrying conductors ltta,'1tib of the horizontal multiple are eifectively coupled to the speech path or rnesasge'carrying conductors 12b, 12C of the verv tical multiple when the bilateral transistors '527, 52,8 are biased 011. y

The principal components of the crosspoint are alpair of electronic switch means 527, 52s which are herefshown as'bilateral transistors. Also included in the crosspoint is amernory device comprising a saturable reactor SR and an electronic switch 559. It is not necessary for the hysteresis loop of the saturable reactor core to have any particular configuration; however, in View o f the avail-V 5219.l The control winding 536 is connected between the SCAN or horizontal control conductor 19d and the vertical control highway 2@ by way of anl isolating diode 531. The control winding 536 is energized when a link applies a ground potential to the vertical control highway Y. 12a and the scanner 417i) applies a pulse to the horizontal controi highway 10d during a time frame which identifies a telephone station in an oit-hooker called condition. Thus, the core of the saturable reactor SR is. driven toward saturation in a first magnetic direction responsive to the simultaneous energization of these two control conductors. v

To control the saturable reactor, the 'pair of secondary windings 538, 539 are connected between (-1-) battery and ground by a series circuit traced through resistors Seti, 543, diodes 542, 541, windings 538, 539, and resistor 547 to ground. 'The Zener diode 541 establishes a threshhold voltage for turning on the thyristor 550 and the rectifying diode 542 prevents the thyristor from turning oit responsive to transients occurringafter it tires. Also included in the series circuit is the current limiting resistor 43 which protects the yZener diode and thyristor.

The resistor 5130 provides an antibias potential for prevent- In the exemplary construction the.

in'gthe thyristor 556 from turning :itself on at elevated temperatures.

To control a switching. of Vthe .thyristor SStlbetween fori and"off conditions, a collector supply potential is ,applied to and removed yfrom`the HOLD conductor 19C under the control, of switch amplifier 511 or thev holdover amplifier 5,18. To turn on the thyristo'r, core flux excursion during saturation induces a pulse in winding 539 to cause the Ybase current tiow that turns on the thyristor.

" Normally, the bilateral transistors 527, are switched oft by Va ground potential applied to their base electrodes (iii) via resistors S45-547 `(with the emitters and collectors referenced to a negative voltage). These transistors are switched onf when the ,thyristor fires because a negative potential on the holdl conductor 16C is applied to the base electrodes (iii) which overcomes the negative voltage from theline transformers, as at winding '504, for example.`

The crosspoint circuit operates inthe following manner. Before subscriber station A goes oft-hook or is called, the

core 532 of the saturable reactor SR is saturated ina magnetic direction determined by the flow of current through the secondary winding S39, the circuit being that traced from ,batteryV through resistor 540, thyristor y550 (the thyristor exhibits Va low ZenerV voltage :from base to emitter), winding 539 yand resistor 547 to ground.

Cyclically and sequentiallyduring Veach time frame t which identifies subscriberA station A the` scannerV 470 pulses conductor 16'and,ftherefore, winding 536 to drive core flux toward, butrnot to, an opposite level-of magnetic saturation. Referring to FIG. 5a, the idle line saturationlevel is arbitrarily identilied as the negative sat-` uration, pointu. The core flux change produced.v by Van appearance ofA the time frame pulse which identifies subscriberstation A is identifiedvbypoint b; After termination of this time frame pulse, current flow through wind-k ing E539 drives the core fiuX back to point a on thehysteresis loop' of FIG. 5a.

winding 536 goes to change thelevel of core iiux; *,therefore, arelati-vely low potential appears onfthe vertical.

control yconductor 12a.k Y

v Means are provided for tiring the crosspoint and storing a memory thereof. station A goes ofi-hook, switchl amplifier 511 negatively' energizes Vthe, HOLD conductor 10c; or, when station During the time framepulsey just described, substantially .all y.energy appliedacross More specifically, when subscriber..

A; is called, the holdover amplifierv 518 negatively ener-1,v

gizes the HOLD conductor 10c. VThe link 13 seleo tively energizes conductor 12a -by grounding it during the time frame thatyidentifies .station- Al and the: control winding 536 is energized.l A pulse is inducedinwinding 538 and thyristor 55) switchesfon. A relatively heavy` current. iiovvsv from ground through ,resistor 547,'

Winding 539,-emitter e,and collector c of thyristor 550' to the negative HOLD conductor 10c. Current flow throughfwinding 539, in Athis direction, aids the current in winding 536 and helps drivecore ux to positive saturation vat point c of FIG. 5a. After saturation, currentlfrom the negative conductor 10c through the winding 539 lholds theV core iiux at point c.- The fired thyristor and positively saturated core constitute a memory oftheV fired crosspoint. Also, the voltage drop across resistor 547 changes with this current flow to switch .onf the bilateral transistors 527, 528. The speech pathormessage carrying conductors 10a, 1Gb are now' effectively coupled to the speech path or message carrying conductors 12C, 12b.

A Ibusy pulse is thereafter transmitted -to the link circuit 13 during each time frame which .identifiesV station A.V Specifically, when the 'l scanner -470 `applies a pulse to conductor 16,A current tiows through diode 531 'and Winding S36 to thevertical control conductor 12a. Since core flux is now at thev level c (FIG. 5a) substantially none of the energy applied bythe scanner 470 goes to o hange therlevel vof core fluxY and a relatively high 'potentialfis appliedto conductor 12a. This high potential is the'busy pulse that preventsia double connection with a single subscriber station..

When the crosspointf fires responsive,` to a1 call to the associated line, vconductor 10c is initially energized by the holdover amplifier 518. answer, conductor 10c is Vdeenergizedvvvhen the ycalling subscriber hangs-up to terminate the seize pulses appliedl `to the LINE SEIZE- HIGHWAY.

nates, the. base electrodes '(iii) of transistors 527,

523 go to the ground supplied through thel load resistor 547 `and bilavteralftranrsistors 527,'j 52S turn .ot`f.1 core iiux of the saturable reactorSR returns to the saturation level (point a) which marks' an idle'crosspoint. f

Ank advantageV of this arrangement', is that the lline v circuit andcrosspoint"matrices'y cooperate to provide an integrated telephoneY system having a l'minimum number of componen-ts. ExtremelyV stalble and reliable magnetic coresvareused to control the crosspoints. Moreover, the busy pulses emanatej fromth'e crosspoint vitself thus allowing a rsimpler and more compact telephone system.r Therefore, the line and controlling link circuits are ,greatly simplified *with a resulting reduction'- in the cost `ofitelephone systems.

i With the foregoingdes'criptions jof `the logic symbols,

the line circuit, and the-crosspoint'in mind, itfis thought that the remainingv structure, features, and yadvantages of the invention ymay be understood bestby a'deseription of arcall asit is extended through the exchange'.` p

Before a call is placed, an allotte19 operatesin any well'known manner -topreassign anidle link (assumed to be linkV 121)')r byjmarkingfthe conductorfdesignated ALLOT '(FIG@ 4). One example of an allotter such as this-maybe foundinttheU.S. patent application entitled t Electronic Switching Telephone System, Serial No. 837,400, filedSeptember` 1,. 1959, Vnow US.' Patent No.

' 3,118,974, by Radcliffe andPickering, and assignedto the kassignee lof thisu invention.v

t. Since; the .station A is idle, theeective impedance of" control winding 536 is relativelyfhighand no busy pulse appears on` kconductor 12a during the timeframe that identifies station A. More specifically, -in. an exemplary system, the scan pulse was' a negative twelve vvolt pulse.

'Ilhere Wasa sixvolt dropvac'ross theWindingY 536fwhile core tiux is, driven from point 'a to point b (FIG. 5a).

The resulting six Voltsapplied over the vertical, control conductor 12141 was not1su1iicient to cause a"br`eakdown diode, here shown asa 'Zenerdiode'Zlgv in the link circuit to conduct. Thus, no busy marking is delivered to OR gate441.lv"

-Fz'nderopevratonr-To place a call,l the subscriberat station A removes; a telephone .handset-or receiver thereby closingv adirectcurrentloop whichmay be traced from negative batterythrough .the .filament of lamp y591, wind-V ing 509, subscriber' set A, and winding .StlZto ground.V Also, a directfcurrentv loop is completedfrom battery throughy the filament of 1amp507; a winding Silea on transformer-TR2; subscriber lset `A,f` the winding 59611,k and the resistor 505 to groundfiV Whenl a voltage is developed across resistor 505; theswitchlamplifier- :511 vturns on Responsive to the ensuing current fiow, the OR` gate 515 conducts tomarka private or sleeve conductor which is also the horizontal control conductor 10c.

Nothingr further happens yuntil the scanner k47d` marks Y If the called party does notA However, Vthis has rno effect be# After the collector supply `is, removed, thyristorA The 9. the output conductor 16 which individually identities the calling subscriber station A.' During the time frame when conductor 16 is marked by the scanner 470, the lower input terminal of AND gate 512 is energized. Since the switch amplifier 511 is also energizing the upper input terminal of AND gate 512, current tlows through the isolating diode 516, thus marking the OFF-HOO highway.

' In the seize inhibit circuity 25 (FIG. 3), the signal on the OFF-HOOK highway is extended through an amplitier 312 to the right-hand terminal of inhibit gate 316. Capacitor 313 charges during each time frame when the "scanner 47'@ pulses conductor 1d and discharges when the pulse is removed, all to maintain the current through the amplifier 312 and the inhibit gate 316.

The potential applied through the inhibit gate 316 and an ampliiier 314 coincides at the input of AND gate 443 with the potential applied over the conductor ALLOT by the allotter Capacitor 315 charges to maintain the input at this. AND gate for a predetermined minimum time period. The AND gate 443 conducts and transmits a signal tothe control terminal C of a gate circuit 442.

Means are provided in the link 4circuit for preparing to iire a crosspoint switch if there is an absence of a busy pulse on the vertical control conductor 12a. This is accomplished in the link at gate circuit 442. which is here shown as including a PNP junction type transistor Q4 having a breakdown or zener diode Z1 connected between its collector and emitter. The base electrode is normally biased positive relative to the emitterl through a resistor R1, thus switching the transistor Q4 oil When the control terminal C is pulsed negative by AND gate 443, the transistor Q4'switches on and currentilows from conductor 12a through the collector- Y emitter circuit to ground through resistor R2. This cur` rent flow. is relatively heavy, thus allowing a greater cur-y rent oW in the control winding 536 and aiding rapid saturation ofrcorev 532. That is core iiuxis driven to point c of FIG.'5a.. As explained above, the thyristor 550 switches on to draw current through winding 539 to help saturate the core. l

Responsive to current flow through transistor Q4, OR gate 441 conducts and a signal is transmitted throughk an ampliiier 440 to a holdoverV amplifier 455 which integrates theV busy pulses that appear on conductor 12a and l@ stantially the full twelve volt output of scanner 470h reaches the zener diode Z1 via conductor 12a during each time frame that identities station A. This twelve volts'exceeds the breakdown voltage of the Zener diode Z1 and the busy pulse is effective at the input of the OR gate 441. Therefore, during each time frame thaty time frame of a busy station and there can be no seizure of station A through the FINDER matrix. This same busy pulse is applied through ampliiier 311, and inhibit gate 320 to the inhibit terminal of gate 334 to prevent seizure of station A through the CONNECTOR matrix.

continuously energizes the lower input terminal of in- I hibit-gate'456. .The'upper or'inhibit terminal of gate:

456 is ynot energized at this time.

potential is extended through the inhibit gate 456 to the i:

Therefore, a signal switch terminal of the tone gate 457. The dial tone whichl is applied in any suitable manner to the input terminal of gate 457 is extended through transformer 351,

to theheavily inked message carrying or talking co'n-A ductor 12b andthe bilateral transistor 528 to the subscriberfstation A.

tion whena call is received.

vMeans are provided for busy marking the link circuit during each time frame which identifies the subscriber station A. More particularly, after the time frame pulse terminates, the lower input terminalvof AND gate 512 is deenergized. The ofehook signal is removed from the OFF-HOOK highway and from the input of AND gate 443 which turns ofi The negative pulse-- applied to the control terminal C of gate lcircuit 442 terminates and transistor Q4 turns ol This is not important, however, because the core 532 has saturated and the impedance of winding 536 has dropped. Thus, sub- Dialing-A subscriber at station A hears the dial tone and proceeds t0 operate any suitable signaling device such as a standard telephone dial to transmit directory information to the link circuit 13. This dial is adapted to transmit a series of open loop pulses over the telephone receive pair of conductors REC extending to the primary windings Stien, 566i: of transformer TR2. Thus, if the subscriber transmits the digit 1, the loop including these windings is opened once; if the subscriber transmits the digit 2, this loop is opened twice, etc. Each dial pulse induces a pulse in the secondary winding 566e of trans? former TR2 and a signal is extended over conductor 1h12, through bilateral transistor 52S, conductorv 12b, conductor 435, and an amplifier 445 to the shift terminal S at each shift register stage. L

As will become more apparent, start gate446 marks theupper or inhibit terminal of gate 456 todeenergize -the switch terminal of dial tone gate 457 which removes dial tone from the line. Also, a delay network 459 is energized to provide approximately 1.2 seconds delay.

- This delay is long enough to ensure the receipt of a complete digit pulse train for a six-line system. Longer delays my be provided in other systems.

vRegistration. The register for storing the dial pulses is here shown as a shift register for six lines; however, thewell known units, tens, and hundreds, etc. decade register will be used in larger systems.

Prior vto the initiation of va call, the reset terminal (marked 'by a heavily inked dot) of each register stageA is v pulsed negatively to reset all stages in the shift register and ground all output terminals marked by a D or '11v Each Vdial pulse applies a positive pulse to all shift terminals 8. Responsive to the first dial pulse, stage RA negatively energizes its output terminal T1 and prepares stage RB. Responsive to the second pulse, stage RA grounds its output terminal T1, While stageRB negatively energizes its output terminal T2 and prepares stage RC. All succeeding stages operate in a similar manner responsive to successive dial pulses. y

The circuits actually used in one exemplary shift reg-l ister are shown in FIG. 4 immediately above the logic symbols. Specilically, these circuits include the start stage 44dand the iirst shift register'. stage RA. It will be apparent that the circuits for each succeeding stage'RB RN are the same as the circuit shown for stage RA.

Each register stage includes a pair of electronic switches which are transistors Q1, Q2, here shown as PNP junction type devices. Thev reset terminals are .shown by heavily inked dots, marked R, and the shift terminals are shown by a circle marked S. The reset pulse is of negative polarity and the shift pulse is of positive polarity, as indicated by the waveforms adjacent the letters R and S respectively. The purpose of the remaining components will be apparent from the following description of the operation of the shift register.

When the negative reset pulse appears at the heavily inked terminalsmarked R the bases of transistor Q1 in the start stage 446 and transistor Q2' in stage RA (and was ott and', therefore,.remains off.;

in each succeeding stage) are made negative'rela'tive to.

its emitter. Thus, these transistorsfturn on.' Referring to stage v446, byl way of example, the emitter ground G1 of transistor Q1 appears at the terminali) and, therefore,

via a voltage divider (resistors R12 and R13) connected' between grounded point D and battery B1. Therefore, transistor Q2 is held oli With'transistorQZ offf negative potentialv is extended through` resistors R16, RTS to the base of transistorv Qi, thus holding it in the on condition originally resulting from the appearance ofthe reset pulse. Also, this same negative potential is applied through resistor 'Rid and capacitor C3 to the base of transistor Q2V which is in the 0n condition that originally resulted from an appearance of the reset pulse. The manner in which transistor Qi is held off wiil be apparent from the foregoing description of the bias applied tothe base of ytransistor Q2.

AWhen arpositive shift pulse appears at each shift terminal S, `transistor Qlswitches olf because its. base goes.

positive ,relative to its emitter. lowever, transistor Qi Qi switches oiff its emitter ground Gli is removed from terminal D,'and the negative potential applied through resistor-R11 appears, in lieu thereof. This neative potential is the signal that inhibits gates 444,456 and ystarts timer:l 459. Also, when transistor Qi Vremoves its emitter ground G1 from terminali), thenegative battery applied The `transistors, Qi, QZy remain in their described states AsV transistor until the reset terminal Ris again energized byV anegative reset pulse to switch transistor Q1 on and, therefore, transistor Q2 olf When transistor `Q2 switches on,,"

its emitter ground-G2 Vappears at terminal TO.` This change in potential at terminal TO (from negative battery to ground) supplies a positive polarity pulse to ,the base of transistor Q2' throughtcapacitor C3. V Since thel hase of transistor QZi goes positive relative toits emitter, it switches off and transistor Q1" switches on`in a mannersimilar to the described switching on of transistor LQ2'.I When transistor QZ' switches offf vits emitter 'ground isV removed from output terminal Ti'and negativeV battery appears in lieu thereof.

i The circuits ofeach of the remaining stages in the shift register are `the same as the circuit of stager RA.4 Thus, whena second positive shift pulse appears at the shift terminals S, thefbase oftransistor Qi' goes positive rela tiveto its emitter; it turnsfotf@ transistor Q2 turns ,on-;

groundgreplaces negative battery at terminal Tilg' and ,a

Y of this description, it is assumed that two digit pulses are received; therefore, the Voutputfor'trigger terminaLTZ Vtransistor in stage RB similar to transistor Q2- switches off to applynegative battery to terminal T2.v In a' l2 Y Y ANDV gate 33t.. The delay circuit 459? delays the ener,- gization of the left-hand terminal of AND gate 331 for a period of time thatrislong enough to ensure the receipt of a complete pulse train. But, 1.2seconds after receipt of the first digit pulse, the left-hand inputterminal of AND gate 331 is energized and when AND gate 453 conducts to energize the right-handinput terminal a pulse is ktransmitted through an amplifier 332 to the -lo'wer input terminal of AND` gate. 34o', the upper input Yterminal of AND gate 342, and theright-hand terminal of inhibit gate 351i. v

Busy `zesz--If station B` is busy, a 'pulse appears on the INPUT BUSY HGHWAY during-the time frame v thatidenties station B and when the right-,hand terminal of AND gate 463 is energized; therefore, the inhibit terminal of gate 334 is energized justy prior to the time when its right-hand terminal is energized owing to Va short delay in 'amplifier 332,'all to prevent the transmission of a pulse to the LINE SEZE Vi-llGI-li/rsr. Also, this same pulse on the INPUT BUSY HIGHWAY is applied to the lower input of ANDA gate 342 to trigger a fiip-iiop 3de which turns on to energizegate 354 and transmit busy tone from conductor BT through trans-v former 351 to the talking conductor iZb and statilo'nA.

Assuming that station B is idle at'the time-described, there is no pulse on the NPUT lBUSY HIGHWAY when AND.` gate 463 conducts. Thus, gate 334 `is not inhibited and a pulser is transmitted through 4the righthand `output of amplifier 335,'and OR gate 321, toian inhibit terminal ,on gate 32). Inhibition of gate 32%) prevents the busy marking (which is about to 'be .returned when the connector.matrixy switches through)`from pre maturelyblocking'gate. 334. The right-hand output of amplifier, 335 alsopulses terminal C of gate 442 and,

therefore; vertical control conductor llSa.' The left-hand.

output of amplifier 335 kis connected to the LINE SEIZE HIGHWAY. which extendslto -oneAND gate (such. as 511.7, 'dtiiiiin each ofthe linecircuits, thus pulsingthe upper input terminaliof .AND gates 517,ttii.1 As will be apparent from the foregoing description fof the line and crosspoint circuits, crosspoint 2l lires.

The rightahand output'terminal of amplifier 3351 also energizes the input; terminal of flip-flip 350, thereby switching on gates.352,353. Responsive thereto, ringing signal currentgflows from source RSI over conductors 434, ftb, and the crosspoint 21 tothe calledy station B where a call signal devicesounds-. Ringback tone is transmitted from source RBT", throughgate 353, 'transformer 351conductor 1211,'transistor V525, conductor 1Gb, and transformer TR2 to subscriberstation A.=1l 'Y Answer supervision. TheY called telephone B .sounds and nothing further happens until'a subscriber removes a handset or receiverto` closer-.a .DC-.loop across-windings 574, 575. Current flows over this closed'loop through current sensing resistor 573 and diodey litiresistor 404,

to charge. capacitor 4135 and'turngfon switcharnplitier 40,2 to energize ,the upper input` terminalr of"ANDy gate ,V 406.- During the time frame when the scanner. 4701is marking output B Aand conductorfla the lower input terminal of ANDWgate 406 is Venergized,and `output current flows through diode. 407 to. the OFF-HOOK HIGHWAY toA charge capacitor 313,:and toapply busy n pulses to gate 3io vand to the upper input terminalof AND gate 345i.k During the time iframe lwhich iden.v vtities the ,called station B,an output currentvr flows from theregister `through'the AND'gate 463,l OR'gate1 of the shift register stage RB is energized at theend of the digit pulse train. Responsive thereto, an energizing currentliows to, the left-hand input terminal on AND gate 463. The scanner 476 is energizing each of the terminals A, B. C N in turn during its individually associated timeframe. Therefore, during the time frame whichidentities subscriber station B, the right-hand terminal of .AND gate 463 is energized totransmit a puise through the OR gate 464 yand an amplilier 33ti'to 464, amplifier 336i, AND gate 331, amplier,332'and thev lower :input terminal of ANDl gate 340.2 VThere is a coincidence at AND gate 340 which triggers a `timer y341.` T1mer 341 turns on and supplies a vpulse of 4a n, predetermined period of time which is adequate to'ensure proper: operation ofthe gate circuits 352, 353.

Thereafter, anenergizing pulse is appliedto the resetv terminal (markedvby a heavily inked dot) of' the flipip 356,;which `turns olf Thus, the tone. gates are? pulse from AND gate 463 cannot get through to ampliiier 335 in'other links. The LINE SEIZE HIGH- WAY is not pulsed and no connector crosspoint can ire.

Release-The call is in progress and nothing further happens until the calling, and called subscribers return their handsets or receivers to a hookswitch. Respongiven time and forspreventing the operation of a link during a time frame which identities a busy station, thereby preventing cross connections between independent calls.

2. An electronic switching` telephone system comprising an electronic crosspoint matrix including intersecting horizontal and vertical multiples, each multiple including at least one voice conductor and one control conductor, `a-plurality of subscriber lines each terminated by a line circuit, each of said line circuits having access to said matrix via an individually associated horizontal multiple, a plurality of link circuits, each of said link circuits having access to said matrix via at least one individually associated vertical multiple, means for prosive thereto, the loop across each receive pair REC is Y broken, current ceases to flow through resistors 50S, 573 and the switch amplifiers 511, 492 turn oi Since the LINE SEIZE HIGHWAY- is not energized after gate 334 is inhibited, the AND gates4ttl, 517y are ofi Reterringspecically to the line circuit 11 by way of example, the iiow of current through switch amplifier 511, ORV gate 515-and the conductor ltic to the collec` tor C of thyristor 550 is interrupted. The thyristor turns oit to deenergize the lower winding 539. Current ows from positive battery through resistor 54), transistor 55h, winding 539, yand resistor 547 to ground. This current 'ow drives the core 532 of saturable reactor SR to mag- BUSY HIGHWA during the time frames that identify. stations A, B, andeither may be seized as described' above. Y,

It Visto be understood that the foregoing description ofA a specific example of the linvention is not to be construed as a limitation upon its scope.

I claim;

1. An electronicswitching telephone systeml comprising an electronic crosspoint matrix including horizontal and vertical voice path and lcontrol path multiples, a plurality of subscriber lines each terminated by a line circuit, eachof said line circuits having access tosaid .matrix via an individually associated horizontal multiple,

a plurality of link circuits, each of said linkcircuits having access to said matrix yvia at least one individually associated vertical multiple, a scanner for producingV a plurality of cyclically recurring time frames, means-for coupling each of said line circuits to said scanner to beenergized during an individually associated timeframe,

means ,for coupling saidscanner to each of said link. circuitsfor sequentially energizing said link circuits during each of said time frames, means in each of said link cir-I cuitslfor selecting a desiredmtime frame in accordance with directory information received during a. given call, an oit-hook highway and a line seize highway common to said line circuits and to said link circuits, means for energizing said olf-hook highway during time frames which identify individual stations when in an olf-hook condition, means in said link circuits for energizing'said lineseize highway and an individually associatedV vertical multipleduring said selected time frame, means responsive to said last named means for completing a circuit through said matrix, and means interposed between said highwaysand said'link circuits for preventing the simultaneousy operation o f more than one link circuit. at any ducing a plurality of cyclically recurring time frames, means for coupiing each of said line circuits to said last named means to energize a control conductor in the associated horizontal multipleV during an individually associated time frame, means in each link circuit for energizing a control conductor in the associated vertical multiple during the time frame which identities a crosspoint that is to ybe fired, electronic switching means and an individually associated memory means at each of said crosspoints, said memory means including a saturable reactor having a coreV and a plurality of windings, one

oi' said windings being connected-between control conductors in the vertical and horizontal multiples that intersect at a crosspoint, whereby the core iiux of the associated saturable reactor is driven to saturation in a first magnetic direction during the timeframe when the control conductors of intersecting horizontal and vertical multiples are energized simultaneously, and means re-` sponsive to the change of impedance in said one winding when said core saturates in said firstmagnetic direction lfor transmitting a busy marking between saidhorizontal controlconductor and said vertical Acontrolconductor which are interconnected by said one winding.

3. An electronic switching telephone system comprising a matrix including a plurality of rst and second groups of conductors arranged to provide intersecting crosspoints, each ofthe groups including message carrying conductors and control conductors, electronic switch means at eachof the crosspoints for eiectiv'elycoupling the message carrying conductors which intersect thereat when said switch means is turned om memoryl means including'a saturable reactor at each 0f saidcrosspoints, each of the reactors including avcore and a plurality of windingsgaV control windingjof each of said reactors connected between the kcontrol conductors' ofsaid first and second groups of lines .which intersect at the individually associated crosspoint, means `responsive to saturation of the core of'said saturable reactor in a first magnetic direction forswitching on, said electronic switching meansk 'at the individually associated crosspoint, and means re-V sponsive to saturation of the core in an opposite magnetic.: direction for 'indicating that said electronic switching.

means is turned ol.

, 4. An electronic Vswitching telephone system comprising a matrix including aplurality ofiirst and second groups-of-'conductors arranged to provide intersecting crosspoints, each ofthe groups including message carrylng conductors and controlconductors, electronic switch means at each of thefcrosspoints for eiectively coupling the messagecarrying conductors which intersect thereat when the associated electronici'switch means is turned 011, a saturable reactor including a core and-a plurality of windings at each of said crosspoints, a control winding of each of said reactors connected between the control conductors of said rs't and second groups of lines which intersect at an individually yassociated one of said crosspoints, meansfor cyclically and sequentially energizing the control conductors of said irst groupsI of conductors,

means .for selectively energizing the control conductor of a particular group of conductors .among said.' second groups of conductors during a preselected time frame toI air/aaai `'winding isfsaturated in a iirst magneticdirection, means responsive to said saturation of the .core in said'rst magnetic direction for switching on said electronic switchf.

k ing rneansassociated therewithmeans responsive to de.y energizing said control conductor of said particulargroupV for'saturating in an Opposite magnetic direction they core Y associated with said energized control winding, and means responsive-to said` last named means for switchlng said electronic switching means.otf.

' 55 An electronic switching telephone system comprising a matrixv including a plurality of iirst and second groups of conductors arranged to provideY intersecting crosspoints, each of the groupsincluding message carrying conductors and control conductors, electronic switch means at each of the-crosspoints for effectively coupling the message carrying .conductors whichv intersect thereat, means including a saturable reactor having la core and a plurality of windings ateach of said crosspointsyeach of said electronic switch. means includes at least one bilateral.

transistor having a .first emitter-collector connected to a messageY carrying conductor in said iirst ,groupfof conductors, .a.secondemitter-collector coupled to a message; carrying conductorin said second group fof conductors, and a'control-,electrode connected to a winding of said.

saturable reactor; a control windingof said s'aturable recoupling ithemiessag'e carrying conductors' which intersect thereat, lmemory means including a magnetic device at each of said crosspoints forcontrolling said electronic de. vices and for transmitting kbusy signals when the crosspoint is closedyand.` means responsive to .a coincidence. of time signalsfor controlling said magnetic and electronic devices if noV busy signals are then present.`

8. Afself-controlled electronic crosspoint for extending message carrying conductors whichgintersect at said crosspointcomprising a saturablereactor. having a core, a control winding,.and atleastone secondary winding, means for cyclically energizing said control winding during partlcular time frames among a number of successively recurring time frames to. biasthe magnetic ux of said core toward but notito magnetic saturation ina rst magneticdirection, means effectivefat the termination ofeach of said cyclic-energizations of said saturating windingior driving thefux of said core backto magneticsaturationv in'an opposite magnetic direction, means responsive to a control signalfor driving said core flux to saturation in .said iirst magnetic direction during Aa selected ,one of said particular time frames, means also responsive' to said control signal for causing said crosspoint tolcomplete a circuit actor connected between the control conductors of said!- irs't and second groups oflines which intersect at across? poinh'means responsive to saturation of the core of said s'aturable reactor in a rst magnetic directionfor switching saidelectronic switching means jon, andmeans `re-v sponsive to .saturationof the core of saidsaturable re-f actor in an opposite magnetic direction for indicating that said electronic switching Ameans is turned oliV between said message carrying conductors which intersect thereat, and, means thereafter. responsive to said 'cyclic' energizations lof said control 'winding for transmitting busy signals indicating thatvsaidjcrosspoint'has fired.

9. 'An electronicv crosspointv comprising a saturable. re-

actor having a core, :a control winding, 'and at least one secondary winding, vmeans for cyclically energizing said'r saturating. winding during particular .time frames to bias the magnetic flux of. said core toward but Vnot tomagnetic v saturation in a rst magnetic direction, means effective at f f6. An electronicswitching telephone system comprising ay matrix including a plurality of irstand second. groups oconductors arrangedto provide intersecting crosspoints,

each of the/groups including message carryingsconductors and control conductors, electronic, switch means-.at each of [the crosspoints for effectively .coupling the'wmessage carrying conductors Whichintersectthereat, means including 4a saturablereactor at each of said crosspoints5 each-of said reactors including a core and a plurality of windings, said'electronic switch means includes at least one bilateral transistor for each intersection of a pair of message carrying conductors, saidv transistors having a tirstjemitter-collector electrode connected to a message carryingconductorin said Yiirstgroup, anda second emit-d ter-collector electrode coupled to amessage carrying con- V ductor in said second group,and a control electrode connected lto awinding ofsaid saturable reactor, a..controly winding of said saturable reactor connected betweenthey control conductors of said iirst andlsecond groups of lines which-intersectatsaid crosspoints, meansA for cyclcallyand "successivelyenergizing said jcontrol conductors of said first groups of conductors, means for selectivelyI ener-v gizinggthe control conductorofone of said second 'groups simultaneously with the energization of oney of said control conductors in a lir'st ofjsaid'groups whereby said core saturates in a first magnetic direction, means responsive to thesaturation of a corefinsaid first magnetic directionA for Iswitching said electronic'switching means on, andk means responsive to saturation of the vcore of said saturabla` reactor inan opposite magnetic direction for indlcatmg thatsaid electronic switching means is turned ofti 7. Anelectronic switching telephone system comprising a matrix'including4 arfplurality of irst and secondu groups of conductors arranged to provideY ,intersectingr cross-points, each of the Ygroups'including message. carrying Aconductorsand control conductors, means including electronic devices .ateach ofthe 'crosspoints for effectively" the termination of each offsaid cyclic energizations of said control winding for driving `Vthe uX-.of saidcore back to magnetic jsaturation in an opposite magnetic.. direction, means responsive to a control signal forfdrivinglsaid'core uX toV saturation in said iir'st direction during a selected' onegof` said time frames, an electronic switch including a' thyristor andat least one bilateral .transistor at 'said'.cr'osspoint; means responsive to core iiux change occurringat said corev saturates in said first direction ,for triggering said lthyristor top-.an .'on.jconditio`n, means. responsive to current owithroughsaid thyristorfor speeding saturation and forthereafter holding said` core' saturated in said first magnetic direction,: means also responsive .to current" now through said thyristor for .switching fon said bilateral'utransistorfand means .responsive tojksaid cyclic. energizations ofv said control winding after Vsaturation of said core in said first direction for'transrnitting'busy sig# nals indicating Vthat saidcrosspoint k.has fired.

l0.=y The electronic'crosspoint kof claim 9 and means for interrupting Vthe ernitterrcollector circuit'of said thyristor to l turn offVY lsaid crosspoint, i and, means` responsive to said thyristoriturning `oi`r" for driving said core iiuxback to saturation in said opposite magnetic direction and for"v switching otf said bilateral, transistor.

' 1111A selfecontrolled"electronic. crosspoint for selec-v tivelyvinterconnecting message carrying conductorswhich intersectl at that crosspointicomprisinga saturable reactor having a core; `acontrol winding,1and atleast one'second'-V ary'winding, means for cyclically energizing said controlv winding during particular time frames among a number` of successively recurringtime framesto bias thelmagnetic flux of said core toward but not to magneticsaturation in' a first` magnetic direction whereby substantially all energyY applied across said controlwinding during saidY particular timeframes goes to changecore flux thus'producinga relatively large voltagedropl across the winding, .means effective'fat the termination'of each of saidcyclic .energizations yof said control Winding for driving the flux of said core back. to magnetic saturation in an opposite magneticiy direction,fmeansresponsive lto a controlrsignal fordriving K saidcore flux tosaturation in said iiirst. magnetic direction during a selected one of said particular time frames series with a parallel circuit, one arm of said parallel circuit including a breakdown diode and the other arm of said parallel circuit including an electronic switch means, means for cyclically energizing said control winding by recurring time frame pulses of uniform voltage, the voltage drop across said control Winding when said reactor is not saturated in a given magnetic direction being such that said breakdown diode does not reach ring potential when energized by said uniform voltage pulses, the voltage drop across said control winding when said reactor is saturated in said given direction being such that said breakdown diode does tire when energized by said uniform voltage pulses, and means responsive to the operation of said electronic switch for sat-urating said reactor in said given magnetic direction.

13. A time controlled switching device comprising a saturable reactor having a control winding connected in series with a parallel circuit, one arm of said parallel circuit including a Zener diode and the other arm of said parallel circuit including an electronic switch means, means for cyclically energizing said control winding by recurring time frame pulses of uniform voltage, the Voltage drop across said control winding when said reactor is not saturated in a given magnetic direction being such that said zener diode does not reach breakdown voltage when energized by said uniform voltage pulses, the Voltage drop across said control winding when said reactor is saturated in said given direction being such that said zener diode does break down when energized by said uniform voltage l@ pulses, means responsive to the operation of said electronic switch for saturating said reactor in said given magnetic direction, and means responsive to saturation of said reactor in said given direction for operating said time controlled switching device.

14. A time controlled switching device comprising a saturable reactor having a control winding connected in series with a parallel circuit, one arm of said parallel circuit including a breakdown diode and the other arm of said parallel circuit including an electronic switch means, means for cyclically energizing said control winding by recurring time frame pulses of uniform voltage, the voltage drop across said control winding when saidreactor is not saturated in a given magnetic direction being such that said breakdown diode does not reach tiring potential when energized by said uniform voltage pulses, the voltage drop across said control winding when said reactor is saturated in said given direction being such that said breakdown diode does lire when energized by said uniform voltage pulses, means responsive to the operation of said electronic switch for saturating said reactor in said given magnetic direction, a thyristor and at least one bilateral transistor, means responsive to saturation of said reactor for triggering said thyristor, means responsive to current ilow through said thyristor for holding said reactor in saturation and for switching on said bilateral transistor, and means responsive to interruption of current ow through said thyristor for switching oit said bilateral transistor and for driving said reactor away from saturation in said given magnetic direction.

References Cited bythe Examiner UNITED STATES PATENTS 2,914,617 11/59 Fritschi et al. 179-18 2,949,504 8/ 60 Rubinoi l79-18.7 2,987,579 6/ 61 Dunlap n 1'79--l8.7

ROBERT H. ROSE, Primary Examiner.

WALTER Lt LYNDE, Examiner. 

7. AN ELECTRIC SWITCHING TELEPHONE SYSTEM COMPRISING A MATRIX INCLUDING A PLURALITY OF FIRST AND SECOND GROUPS OF CONDUCTOR ARRANGED TO PROVIDE INTERSECTING CROSS-POINTS, EACH OF THE GROUPS INCLUDING MESSAGE CARRYING CONDUCTORS AND CONTROL CONDUCTORS, MEANS INCLUDING ELECTRONIC DEVICES AT EACH OF THE CROSSPOINTS FOR EFFECTIVELY COUPLING THE MESSAGE CARRYING CONDUCTORS WHICH INTERSECT THEREAT, MEMORY MEANS INCLUDING A MAGNETIC DEVICE AT EACH OF SAID CROSSPOINTS FOR CONTROLLING SAID ELECTRONIC DEVICES AND FOR TRANSMITTING BUSY SIGNALS WHEN THE CROSSPOINT IS CLOSED, AND MEANS RESPONSIVE TO A COINCIDENCE OF TIME SIGNALS FOR CONTROLLING SAID MAGNETIC AND ELECTRONIC DEVICES IF NO BUSY SIGNALS ARE THEN PRESENT. 